This invention relates to an automatic tracking control system for an apparatus that uses one or more pickup heads to reproduce signals recorded on a plurality of mutually adjacent tracks. Examples of such apparatus include video tape recorders (VTRs) and video cassette recorders (VCRs), which reproduce video signals recorded on a magnetic tape.
A VTR or VCR generally comprises two or more video heads that are rotationally driven at high speed while the magnetic tape is transported past them, causing the video heads to scan slanted tracks on the tape. The tape speed is controlled by a servo loop that keeps the heads positioned over the centers of the tracks by establishing a certain tracking delay between a pulse signal representing the rotational position of the heads and a control signal representing the position of the tracks. The tracking delay is controlled by a tracking control signal input to the servo loop. In an automatic tracking control system the tracking control signal is generated by an automatic control circuit that monitors the video signal reproduced by the video heads.
In prior-art automatic tracking control systems the automatic control circuit attempts to maximize the envelope of the output from the video head amplifier. A problem with such automatic tracking control systems is that the video heads pick up not only the signal from the intended track but also crosstalk interference from adjacent tracks, which has a destabilizing effect on automatic tracking, degrading the signal-to-noise ratio of the reproduced video signal.
Crosstalk is particularly troublesome when a tape is recorded on one machine and played back on another machine, a common mode of usage of VCRs. Many VCRs are also built with video heads wider than the video tracks, for special playback modes such as still-or slow-motion in which accurate tracking is not possible, and wide heads further aggravate the crosstalk problem. Due to crosstalk, automatic tracking control systems of the prior-art type have generally proven impractical for VCRs.